WIP: Skip navigation button

[je-boska]

This is an accessibility feature. Users who tab through the page elements, can skip the navigation by clicking this otherwise hidden button as they start tabbing.

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[je-boska]

@rcos7 Could you implement this as a part of the Substrate plugin instead?

[je-boska]

DEPRECATION NOTICE Due to changes in the site architecture, this feature will be implemented elsewhere, with this PR used for reference only.